1. Field of the Invention
The invention relates to a method and an apparatus for the treatment of radioactive evaporator concentrates from the evaporation system of nuclear plants, which allow efficient separation, as a non-radioactive product, of the sodium sulfate contained therein and, thus, a cost-effective reduction in volume of the concentrates to be decontaminated and disposed.
2. Description of the Related Art
In nuclear power plants comprising a boiling water reactor, the main condensate is freed from solid ionic and radiochemical contaminations by means of ion exchange resins. During regeneration of said ion exchange resins with caustic and sulfuric acid, solutions are produced which, after having been neutralized, are conveyed into an evaporating plant, where they are concentrated together with other radioactively loaded effluents to a solids content of about 20%. Thus, depending on the size of the plant, about 20 to 100 m.sup.3 of evaporator concentrates per year are obtained as liquid radioactive waste.
The evaporator concentrates contain about 1 to 30% by volume of separable solids (ion exchange resin residues, flocculated detergent residues, heavy metal oxides, fibers etc.). Dissolved components are mainly sodium sulfate, the proportion of which may be about 10 to 25% by weight, and other components, such as incrustation inhibitors (e.g. EDTA), surfactants, organic and inorganic salts of decontaminating agents (e.g. citrates, oxalates, phosphates), activation products and others.
In the past, these evaporator concentrates had been further evaporated in a subsequent process step for conditioning and solidified to form a monolithic sodium sulfate block which then proceeded to ultimate waste disposal or interim storage.
This, however, has the disadvantage that vast volumes of waste are produced and no decontamination effect is achieved, since radioactive components are encapsulated in the crystals when the sodium sulfate crystallizes. Although 80% or more of the mass to be ultimately disposed of consists of non-radioactive sodium sulfate, according to current practice the latter has to be disposed of in a rather expensive way together with the radioactive components.
The suggestions made so far for improving the treatment of evaporator concentrates and similar effluents from nuclear plants were directed to binding the effluent's radioactivity by suitable precipitation or flocculation reactions in such a way that the radioactive products can be removed. The remaining decontaminated solution should then be disposed of as weakly radioactive effluent. The disadvantages thereof were the bad precipitation efficiency, so that said process could not be used in modern nuclear plants due to the associated increase in radioactive release.